A groundbreaking new study found zero transmissions occurred between HIV-positive men with an "undetectable viral load" due to treatment, and their HIV-negative partners, across thousands of instances of anal sex without a condom.

The Opposites Attract study, led by professor Andrew Grulich from the Kirby Institute, followed a cohort of 358 gay male couples – one partner HIV-positive, the other HIV-negative – in Australia, Thailand and Brazil.

The HIV-positive partners in the study had an "undetectable viral load", meaning they are on treatment to suppress the virus so it is undetectable in the blood.

Not a single HIV transmission occurred across the almost 17,000 times participants reported having anal sex without a condom.

12,000 of those sexual encounters were protected solely by the HIV-positive partner's undetectable viral load, and in the other 5,000, the HIV-negative partner was also taking a drug to protect against contracting HIV, known as PrEP.

“It really does confirm that undetectable viral load is completely effective at preventing transmissions in gay couples," Grulich told BuzzFeed News from Paris, where he is presenting the research to the International Aids Society (IAS) Conference on HIV Science.

"Essentially, we’re documenting that this is a form of safe sex for couples in this situation."

Grulich described the discovery as "transformative" for gay male couples with differing HIV status.

"This tells us that if an HIV-positive person gets on treatment, achieves undetectable viral load – and that can take three-to-six months, it’s not immediate – they will not transmit, provided they continue to take their pills daily and see their doctor to have their viral loads monitored."

Grulich said previous data had been "very very firm" in showing that an undetectable viral load prevents transmission in heterosexual couples, but more limited when it came to the same effect in gay male couples.

"There was a real concern that because anal transmission is a much higher risk behaviour that vaginal transmission, that the results may not apply to gay men, or to anal sex in particular. This is great news, that it is in fact as protective for anal sex as it is for vaginal sex."

Opposites Attract is the largest study into HIV transmission between gay couples of differing HIV status ever undertaken, and the first to consider data from both high-income and middle-income countries.

It ran for four years between 2012 and 2016, following each couples for about one and a half years.

In a media release about the study, workers in the HIV and health sectors in Thailand, Brazil and Australia welcomed the research and its implications.

Dr Beatriz Grinsztejn from the Evandro Chagas Clinical Research Institute in Brazil said the study would help break down the stigma around living with HIV: "These results strengthen the argument for treatment as prevention and provide couples with options when it comes to negotiating safe sex."

Dr Nittaya Phanuphak, chief of the Prevention Department at the Thai Red Cross AIDS Research Centre, also said the research would "heavily de-stigmatise" HIV-positive Thai gay men and their partners.

Brent Allen, CEO of Living Positive Victoria, said the results could "change not only the ways in which many people living with HIV view themselves, but importantly how others view people living with HIV".

"People living with HIV have accepted the stinging pain of rejection and discrimination for years," he said.

"Now, based on proven scientific evidence, we can let go of some of the fear and anxiety and feel confident that the sex we negotiate with our partners cannot inadvertently result in an HIV infection.

"The challenge ahead is to get this information and the effectiveness of treatment as prevention into the minds of the community and dismantle the stigma that continues to thwart our efforts to get people to test frequently and start treatment early."

Lane Sainty is a reporter for BuzzFeed News and is based in Sydney, Australia.